The present invention relates to a battery power supply system (battery system) for supplying electric power to a battery operated apparatus.
Recently, there is increased demand for portable personal computers, which can be operated by a battery. The personal computer designed to set importance on small size and light weight thereof has low power consumption and can be operated by a single small battery, while the personal computer designed to set importance on performance thereof has large power consumption and must be operated by one large battery or two medium batteries. When two batteries are used, there is an advantage that one of the two batteries can be replaced while the personal computer is operated by the other battery. In the prior art in which the two batteries are used, as disclosed in Japanese Patent Unexamined Publication No. 2-23045, a wired-OR circuit configuration of diodes is used to supply electric power from a plurality of power supplies. FIG. 5 shows a circuit configuration described in the above-mentioned Publication No. 2-23045, in which numerals 31 and 32 denote power supplies, 33 a diode connected in a path for supplying electric power of the power supply 31, 34 diodes connected in a path for supplying electric power of the power supply 32, and 35 a load. The number of diodes is varied to produce different voltages so as not to supply electric power by both of the power supplies 31 and 32 simultaneously. When the number of power supplies is further increased, the number of diodes is further increased, so that output voltages of the power supplies necessarily differ from each other.
In the above-mentioned prior art, electric power of the battery is supplied through the diode, and power consumption and generation of heat by a voltage drop across the diode are not considered. The personal computer requiring large current has large power consumption by the voltage drop, which can not be neglected. Further, the above-mentioned prior art also did not consider the charging of a plurality of batteries, and the battery system thereof is incomplete.